1. Field of the Invention
The present invention relates to an optical tunable filter and a method of manufacturing the same.
2. Related Art
The following applications relate to the optical tunable filter according to the present invention.
Filter by Surface Micro-machining Technology
In a conventional optical tunable filter, the thickness of a sacrificial layer solely controls the thickness of a variable gap therein. According to this technology, however, the thickness of the sacrificial layer widely varies depending on forming conditions. This can lead to a problem wherein driving becomes unstable because the thickness variation makes a coulomb force (electrostatic force) between a thin film and a drive electrode variable. Furthermore, the thickness of the optical tunable filter is large since a movable part protrudes from a substrate (for example, see Japanese Unexamined Patent Publication No. 2002-174721).
Filter Using SOI Wafer
In U.S. patent document U.S. Pat. No. 6,341,039, a SiO2 layer of a Silicon on Insulator (SIO) wafer is used as the sacrificial layer and forms a variable gap. In this way, the variable gap can be well controlled when it is formed.
However, there is a problem wherein the movable part and the drive electrode can stick to each other when a large electrostatic force is generated. This happens because an insulating structure is not formed between the movable part and the drive electrode (for example, see U.S. Pat. No. 6,341,039).
Problems Shared by the Above-mentioned Filters
At the end of the above manufacturing processes, the sacrificial layer is released and the variable gap is formed. In the formation process, a release hole is needed to introduce a liquid for releasing the sacrificial layer. This decreases an area to which the coulomb force acts which causes a problem in that it increases a drive voltage.
Moreover, when the variable gap is small, a phenomenon wherein the thin film and a drive electrode substrate stick to each other with surface tension of water as the sacrificial layer is released can occur.
Furthermore, when the overall thickness of the optical tunable filter is made thin in order to reduce the drive voltage, an antireflection film formed on the movable part might be warped. Also, the stress induced warping of the high reflection film might occur at the movable part. In addition, there is a problem in that the strength of the optical tunable filter might be reduced.
For these reasons, there is a need for an optical tunable filter in which the movable part can be stably driven with a low voltage and without making the variable gap small.
The present invention has been developed in consideration of the above-mentioned problems, and intended to provide an optical tunable filter in which the movable part can be stably driven with a low voltage and having a simplified structure and manufacturing process as well as being downsized. The present invention also intends to provide a method of manufacturing the same.